In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
Marking particle material is very cohesive which can readily agglomerate in hoppers to produce structures commonly referred to as stable ratholes and bridges. Such stable ratholes or bridges prevent uniform delivery of marking particles to the exit point from the hopper. This results in variations in concentration of the marking particles in the development station of the reproduction apparatus which can ultimately lead to image defects in the copies made by the reproduction apparatus. Marking particles also tends to stick to surfaces (even vertical surfaces) of the hopper making sensing the level of marking particles in the hopper difficult.
The marking particle material can form agglomerates or flakes through either cohesive or adhesive forces. Marking particle agglomerates formed cohesively are classified as hard or soft. Hard agglomerates cannot be broken up by the action of development station mixing, while soft agglomerates can be so broken up. On the other hand, marking particle flakes are pieces of melted or softened marking particles that have adhered together and hardened. Both the agglomerates and the flakes can be transferred to the dielectric support member of the reproduction apparatus and result in the formation of unwanted (undesirable) artifacts, or spots of marking particles, on copies made by the reproduction apparatus that render the copies unacceptable.
Current technology for preventing agglomeration of particulate material normally involves the use of a plastic or metal hopper with internal mechanical mixing elements such as stirring rods, oscillating bars, or rotating wire cages. These mixing elements serve to break up ratholes and bridges and also keep the marking particles moving towards the hopper exit. Additionally, the marking particle material hoppers generally include level sensors to monitor the level of particulate material in the hopper so as to determine when the material has to be replenished. Level sensors may include piezoelectric, capacitive, or inductive proximity sensors. The internal mechanical mixing elements and/or additional mechanical wipers are used to clear marking particles away from level sensors for accurate level reading. However, these internal mechanical mechanisms, whether being used to break up agglomerates or clear the level sensors, can in and of themselves cause flakes and agglomerates.
An alternative to internal mechanical mixing elements for preventing particulate material from agglomerating involves the use of flexible walls for the particulate material-containing hopper. Such flexible wall technology has been used in other industries before for feeding large amounts of cohesive materials. These flexible wall hoppers generally have a paddle on either side thereof. The paddles pivot from their midpoints to flex the flexible walls in order to break up the ratholes and bridges. The flexible wall hoppers in common commercial use now appear to rely on large paddle actuations to insure good particulate material flow. However, such paddle action is not suitable for use in marking particle material hoppers due to compression of the marking particles in the bottom part of the hopper. This would result in agglomerate and flake production. Moreover, if the paddle action were merely reduced so as to provide as little mechanical intervention as possible to reduce agglomerate and flake creation, this low level of actuation would fail to move the cohesive marking particles enough to completely break up bridging, in particular, at the pivot point of the paddle.